Molecular dynamics simulations of the liquid-vapor interface of a molten salt. III. Size asymmetry effects and binary mixtures

The structure and surface tension of the liquid-vapor interface of pure LiCl and KCl molten salts and of LiCl-KCl binary mixtures of different concentrations have been studied by using molecular dynamics simulations and a slab geometry. For the surface tension and its temperature and composition dependences, good agreement with experimental determinations is achieved. A comparison of the structure of LiCl and KCl interfaces allows us to discuss the relevance of size asymmetry effects on interfacial properties. These effects are apparent in the structure of the LiCl interface, but only when a description of polarization interactions is included in the potential model. Polarization facilitates the formation of a displacement dipole moment across the interface by allowing an opposing electronic dipole moment of approximately the same magnitude, so that the total dipole moment is always small and thus not unfavorable from an electrostatic point of view. The study of the binary mixtures shows that the surface tension is relatively insensitive to the specific LiCl concentration for a broad range of compositions. This is explained by potassium segregation towards the surface. In these cases, where segregation effects are important, we have found that very long simulations (several nanoseconds long) are needed in order to obtain converged density profiles. For all the systems studied, the local ion coordination number reduces to 50% of its bulk value only when the ion density has reduced to less than 5% of its bulk value, showing the importance of clustering effects in this kind of interface. (C) 2002 American Institute of Physics.